Lung cancers, especially the small-cell type, are sometimes associated with profound muscle weakness referred to as the myasthenic syndrome. Recently, we found that the lung cancer tissue obtained from a patient with the syndrome contained a "myasthenic substance" which reproduced the characteristics of the in vivo syndrome in an in vitro experimental condition. We assume that this "myasthenic substance" is produced by lung cancers of the small-cell type in general and cause the myasthenic syndrome in some patients. The present project is designed to determine the pathophysiological mechanism responsible for this unusual association of lung cancers with a neuromuscular disorder. The specific issues to be studied are: 1. Systematic neurological evaluation of lung cancer patients to determine whether they have the myasthenic syndrome. 2. Neurophysiological bioassay of the "myasthenic substance" in the serum and cancer tissue of lung cancer patients. 3. Further neurophysiological and biochemical characterization of the "myasthenic substance." It is hoped that the information obtained from the present investigation will result in the development of a new diagnostic tool for lung cancers.